It is often necessary to radiate a complex signal, such as one containing multiple carriers, from a single antenna, with each carrier requiring high power to adequately service a wide area. Carriers within a narrow bandwidth may be combined at low level and then transmitted through a common power amplifier; however the amplifier must be linear to preserve the essential information contained in the envelope of the complex signal produced by vector addition of the multiple signals. Any nonlinearity in the amplifier transfer function can result in spurious products, which can sometimes be suppressed by filtering, and cross modulation interference between signals, generally a degradation which cannot be remedied by further processing. Linearity requirements apply to multiple angle-modulated signals, e.g. frequency-modulated and phase-modulated, and even more stringently to amplitude-modulated signals.
Nonlinear power amplifiers, especially when operated in a saturated mode, are a typical example of nonlinear systems which are subject to inherent transmission limitations. These amplifiers can be made inexpensive and power-efficient, however, generally the use of this class of amplifier must be confined to a single isolated angle-modulated or pulse-modulated signal. Where multiple source signals are angle-modulated and sufficiently separated in frequency, they may be amplified in separate nonlinear amplifiers, each dedicated to a channel, and then combined in either a passive high power multiplexer or a power combiner; however such systems tend to be complex and expensive, and would not be applicable to the class containing multiple signals which are closely spaced in frequency within a narrow band and which, in some instances, are amplitude-modulated. These signals are within the class of complex signals addressed by the present invention.
Linear amplifiers typically require enhancement techniques such as feedback and feed forward, (some systems utilizing active feed forward, that is, amplifiers having pilot tone cancellation). Nevertheless, despite its high cost and low power-efficiency, linear power amplification remains the basic conventional approach for handling multiple signals, and is often utilized due to lack of a viable alternative.